Help: Reorganise target_precompile_headers() docs for readability

Incorporates clarification of behavior when trying to specify
REUSE_FROM on a target that already has its own precompile
headers set.

Fixes: #19970

Help/command/target_precompile_headers.rst

@@ -3,33 +3,21 @@ target_precompile_headers
 
 Add a list of header files to precompile.
 
+Precompiling header files can speed up compilation by creating a partially
+processed version of some header files, and then using that version during
+compilations rather than repeatedly parsing the original headers.
+
+Main Form
+^^^^^^^^^
+
 .. code-block:: cmake
 
   target_precompile_headers(<target>
     <INTERFACE|PUBLIC|PRIVATE> [header1...]
     [<INTERFACE|PUBLIC|PRIVATE> [header2...] ...])
 
-  target_precompile_headers(<target> REUSE_FROM <other_target>)
-
-Adds header files to :prop_tgt:`PRECOMPILE_HEADERS` or
-:prop_tgt:`INTERFACE_PRECOMPILE_HEADERS` target properties.
-
-The second signature will reuse an already precompiled header file artefact
-from another target. This is done by setting the
-:prop_tgt:`PRECOMPILE_HEADERS_REUSE_FROM` to ``<other_target>`` value.
-The ``<other_target>`` will become a dependency of ``<target>``.
-
-.. note::
-
-  The second signature will require the same set of compiler options,
-  compiler flags, compiler definitions for both ``<target>``, and
-  ``<other_target>``. Compilers (e.g. GCC) will issue a warning if the
-  precompiled header file cannot be used (``-Winvalid-pch``).
-
-Precompiling header files can speed up compilation by creating a partially
-processed version of some header files, and then using that version during
-compilations rather than repeatedly parsing the original headers.
-
+The command adds header files to the :prop_tgt:`PRECOMPILE_HEADERS` and/or
+:prop_tgt:`INTERFACE_PRECOMPILE_HEADERS` target properties of ``<target>``.
 The named ``<target>`` must have been created by a command such as
 :command:`add_executable` or :command:`add_library` and must not be an
 :ref:`ALIAS target <Alias Targets>`.
@@ -38,32 +26,9 @@ The ``INTERFACE``, ``PUBLIC`` and ``PRIVATE`` keywords are required to
 specify the scope of the following arguments.  ``PRIVATE`` and ``PUBLIC``
 items will populate the :prop_tgt:`PRECOMPILE_HEADERS` property of
 ``<target>``.  ``PUBLIC`` and ``INTERFACE`` items will populate the
-:prop_tgt:`INTERFACE_PRECOMPILE_HEADERS` property of ``<target>``.
-(:ref:`IMPORTED targets <Imported Targets>` only support ``INTERFACE`` items.)
-Repeated calls for the same ``<target>`` append items in the order called.
-
-Arguments to ``target_precompile_headers`` may use "generator expressions"
-with the syntax ``$<...>``.
-See the :manual:`cmake-generator-expressions(7)` manual for available
-expressions.  See the :manual:`cmake-compile-features(7)` manual for
-information on compile features and a list of supported compilers.
-The ``$<COMPILE_LANGUAGE:...>`` generator expression is particularly
-useful for specifying a language-specific header to precompile for
-only one language (e.g. ``CXX`` and not ``C``).
-
-Usage
-^^^^^
-
-.. code-block:: cmake
-
-  target_precompile_headers(<target>
-    PUBLIC
-      project_header.h
-      "$<$<COMPILE_LANGUAGE:CXX>:cxx_only.h>"
-    PRIVATE
-      [["other_header.h"]]
-      <unordered_map>
-  )
+:prop_tgt:`INTERFACE_PRECOMPILE_HEADERS` property of ``<target>``
+(:ref:`IMPORTED targets <Imported Targets>` only support ``INTERFACE`` items).
+Repeated calls for the same ``<target>`` will append items in the order called.
 
 The list of header files is used to generate a header file named
 ``cmake_pch.h|xx`` which is used to generate the precompiled header file
@@ -79,6 +44,26 @@ must be available for the compiler to find them.  Other header file names
 source directory (e.g. :variable:`CMAKE_CURRENT_SOURCE_DIR`) and will be
 included by absolute path.
 
+Arguments to ``target_precompile_headers()`` may use "generator expressions"
+with the syntax ``$<...>``.
+See the :manual:`cmake-generator-expressions(7)` manual for available
+expressions.  See the :manual:`cmake-compile-features(7)` manual for
+information on compile features and a list of supported compilers.
+The ``$<COMPILE_LANGUAGE:...>`` generator expression is particularly
+useful for specifying a language-specific header to precompile for
+only one language (e.g. ``CXX`` and not ``C``).  For example:
+
+.. code-block:: cmake
+
+  target_precompile_headers(myTarget
+    PUBLIC
+      project_header.h
+      "$<$<COMPILE_LANGUAGE:CXX>:cxx_only.h>"
+    PRIVATE
+      [["other_header.h"]]
+      <unordered_map>
+  )
+
 When specifying angle brackets inside a :manual:`generator expression
 <cmake-generator-expressions(7)>`, be sure to encode the closing ``>``
 as ``$<ANGLE-R>``.  For example:
@@ -88,13 +73,38 @@ as ``$<ANGLE-R>``.  For example:
   target_precompile_headers(mylib PRIVATE
     "$<$<COMPILE_LANGUAGE:C>:<stddef.h$<ANGLE-R>>"
     "$<$<COMPILE_LANGUAGE:CXX>:<cstddef$<ANGLE-R>>"
-    )
+  )
+
+
+Reusing Precompile Headers
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The command also supports a second signature which can be used to specify that
+one target re-uses a precompiled header file artefact from another target
+instead of generating its own:
+
+.. code-block:: cmake
+
+  target_precompile_headers(<target> REUSE_FROM <other_target>)
+
+This form sets the :prop_tgt:`PRECOMPILE_HEADERS_REUSE_FROM` property to
+``<other_target>`` and adds a dependency such that ``<target>`` will depend
+on ``<other_target>``.  CMake will halt with an error if the
+:prop_tgt:`PRECOMPILE_HEADERS` property of ``<target>`` is already set when
+the ``REUSE_FROM`` form is used.
+
+.. note::
+
+  The ``REUSE_FROM`` form requires the same set of compiler options,
+  compiler flags and compiler definitions for both ``<target>`` and
+  ``<other_target>``.  Some compilers (e.g. GCC) may issue a warning if the
+  precompiled header file cannot be used (``-Winvalid-pch``).
 
 See Also
 ^^^^^^^^
 
-For disabling precompile headers for specific targets there is the
-property :prop_tgt:`DISABLE_PRECOMPILE_HEADERS`.
+To disable precompile headers for specific targets, see the
+:prop_tgt:`DISABLE_PRECOMPILE_HEADERS` target property.
 
-For skipping certain source files there is the source file property
-:prop_sf:`SKIP_PRECOMPILE_HEADERS`.
+To prevent precompile headers from being used when compiling a specific
+source file, see the :prop_sf:`SKIP_PRECOMPILE_HEADERS` source file property.